Learning Outcomes:
- Understand the composition and function of blood and lymph.
- Identify the role of the heart and blood vessels in circulation.
- Recognize the significance of blood groups and Rh factor.
- Comprehend the cardiac cycle and the role of pacemakers.
- Examine disorders related to the circulatory system.
All living cells must receive nutrients, oxygen, and other essential substances, while waste products must be removed for optimal tissue function. Various organisms have developed mechanisms for the efficient transport of these substances. In simpler organisms like sponges and coelenterates, water from their surroundings circulates through body cavities, enabling cell exchanges. More complex organisms rely on specialized body fluids for transport, such as blood and lymph. This chapter explores the composition and functions of blood, lymph, and the mechanisms that circulate these fluids.
Blood is a specialized connective tissue composed of a fluid matrix called plasma and formed elements. It plays a crucial role in transporting gases, nutrients, and waste products.
Plasma is a straw-colored, viscous fluid that makes up about 55% of blood volume. It consists of:
The formed elements include erythrocytes, leucocytes, and platelets, collectively making up 45% of the blood.
Blood differs between individuals, necessitating the identification of specific blood groups. The two most common systems are ABO and Rh.
ABO grouping is based on the presence or absence of surface antigens A and B on RBCs. Corresponding antibodies exist in plasma, and blood transfusion compatibility is crucial to avoid clumping. The four blood types are A, B, AB, and O.
Important Note: People with O blood are called universal donors as their blood can be donated to anyone, while those with AB blood are universal recipients because they can receive blood from any group.
The Rh antigen is present in about 80% of humans. Individuals with the antigen are Rh-positive, while those without it are Rh-negative. Rh compatibility is essential during blood transfusions and pregnancy. Erythroblastosis foetalis can occur if an Rh-negative mother has an Rh-positive fetus, but it can be prevented by administering anti-Rh antibodies.
Blood coagulation or clotting is a critical process to prevent blood loss following an injury. It involves the transformation of fibrinogen to fibrin, forming a clot. This process is initiated by thrombin, which activates fibrinogen, and thrombokinase, an enzyme complex, facilitates this conversion. Calcium ions also play a pivotal role in clotting.
As blood flows through capillaries, some water and small molecules pass into the interstitial fluid. This fluid, similar in composition to plasma, allows for the exchange of nutrients and gases between blood and cells. The lymphatic system collects and drains this fluid back into the venous system. Lymph, which contains specialized lymphocytes, is crucial for immune responses and acts as a carrier of nutrients and fats.
Important Note: Fats are absorbed through lymph vessels called lacteals in the intestinal villi.
Circulation in organisms can follow either an open or closed system.
All vertebrates have a chambered heart. Fishes possess a two-chambered heart with an atrium and a ventricle, resulting in single circulation. Amphibians and reptiles (except crocodiles) have a three-chambered heart, where blood partially mixes. Crocodiles, birds, and mammals have a four-chambered heart, ensuring double circulation—complete separation of oxygenated and deoxygenated blood.
The human circulatory system consists of a heart, a network of blood vessels, and blood. The heart is a mesodermal organ located in the thoracic cavity and has four chambers: two atria and two ventricles.
An electrocardiogram (ECG) records the electrical activity of the heart. The P-wave indicates atrial depolarization, the QRS complex represents ventricular depolarization, and the T-wave signifies ventricular repolarization. Deviations in the ECG can indicate cardiac abnormalities.
Important Note: The ECG is an essential tool for diagnosing heart conditions.
Humans have double circulation, involving pulmonary and systemic circulation. Pulmonary circulation carries deoxygenated
blood from the right ventricle to the lungs for oxygenation. The systemic circulation distributes oxygenated blood from the left ventricle to the body tissues, where deoxygenated blood is collected and returned to the heart.
Pulmonary Circulation | Systemic Circulation |
---|---|
Transports deoxygenated blood to the lungs. | Supplies oxygenated blood to body tissues. |
Begins at the right ventricle. | Starts at the left ventricle. |
Ends at the left atrium. | Ends at the right atrium. |
The heart is myogenic, meaning its activity is regulated by nodal tissues. However, the autonomic nervous system (ANS) can modulate the heart rate:
Several conditions affect the circulatory system:
MCQ: What is the primary function of the sinoatrial node (SAN)?
Answer: It acts as the pacemaker, initiating the heart’s rhythmic contractions.